Over the past ten years there has been a rapid growth in the market for linear low density polyethylene (LLDPE). A broad range of LLDPE's are now used in injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, usually about 3 to 10 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer and the catalyst type used in the polymerization.
Many LLDPE resins typically incorporate 1-butene or 1-hexene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethylene/1-butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-1-pentene, and 1-octene. The bulk of the LLDPEs manufactured today are used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes them well suited for a broad spectrum of applications. LLDPE films are often characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.
These properties are generally enhanced by increasing the molecular weight of the resin. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly the extrudability can be improved.
Films typically used in the construction industry are generally required to exhibit toughness, processability and good water vapor transmission resistances (WVTR) sometimes referred to as moisture vapor transmission resistance (MVTR).
Today one can improve water vapor barrier properties of films by either using high density polyethylene (HDPE) resins or using engineering polymers like EVOH and Nylon materials. Each of these materials has their own drawbacks. HDPE's have poor abuse (for example, dart impact) properties and so for applications such as films for use in the constructions industry, where dart impact property is needed in addition to low WVTR, HDPE's don't work well. One could use linear low density polyethylene (LLDPE) resins for improved dart properties, but they have poor WVTR properties. Special equipment (multilayer/coex) and costly tie layer resins are needed for adding EVOH or Nylon layers to films and so they have equipment/economic limitations. Accordingly, there is a need for a solution that would allow production of a film with better balance of low WVTR and high dart that can be made on standard (monolayer) equipment without need for high cost tie resins.
The present invention relates to a monolayer film comprising a mixture of a first linear polyethylene having a density in the range of from 0.926 to 0.970 g/cm3; a second LLDPE having a density in the range of from 0.868 to 0.920 g/cm3; and a nucleating agent to get to a better balance of low WVTR and dart while still allowing film production from it on standard monolayer equipment without need for co-extrusion capability. The LLDPE with the higher density is further characterized by having at least 57 percent (by weight of that component) of molecules having a weight average molecular weight in the range of 31,000 g/mole to 1,000,000 g/mole. These components will be blended in various ratios ranging from 50 to 90% of the higher density LLDPE, 10 to 50% of the lower density LLDPE and at least 50 ppm of the nucleating agent. Preferably the film will have a thickness from 1-20 mil, and an overall density in the range of from 0.925 to 0.939 g/cm3.
The inventive films are well suited for construction film (such as a film for use between soil and concrete in building foundation).